Mine closure of pit lakes as terminal sinks: best available practice when options are limited?

In an arid climate, pit lake evaporation rates can exceed influx rates, causing the lake to function as a hydraulic terminal sink, with water levels in the pit remaining below surrounding groundwater levels. We present case studies from Western Australia for two mines nearing closure. At the first site, modelling indicates that waste dump covers for the potentially acid forming (PAF) material would not be successful over the long term (1,000 years or more). The second site is a case study where PAF management is limited by the current waste rock dump location and suitable cover materials. Pit lake water balance modelling using Goldsim software indicated that both pit lakes would function as hydraulic terminal sinks if not backfilled above long-term equilibrium water levels. Poor water quality will likely develop as evapoconcentration increases contaminant concentrations, providing a potential threat to local wildlife. Even so, the best current opportunity to limit the risk of contaminant migration and protect regional groundwater environments may be to limit backfill and intentionally produce a terminal sink pit lake

The Future Direction of Pit Lakes: Part 2, Corporate and Regulatory Closure Needs to Improve Management

AbstractPit lakes may present significant risks to ecological and human receiving environments but can also provide beneficial end use opportunities. The understanding of many processes that influence the magnitude of these risks and opportunities remains limited, and even where our understanding is adequate, the application of that knowledge is not consistently applied. From initial planning to long-term closure, regulation and corporate management of pit lake closure can be improved to realise more sustainable pit lake legacies. In this two-part manuscript, we recommend focus areas for future research by academics (Part 1), and strategies to structurally improve the practice of pit lake closure for mining industry regulators, corporate sustainability officers, global practice leads, and site mine closure planners (Part 2). Here we identify barriers that often limit the understanding of pit lake processes and closure practices and suggest ways that corporate leaders, closure practitioners, and regulators can improve pit lake management. Recommended corporate changes include: conducting risk assessments at an early planning stage; funding pit lake research and trials; allowing data sharing and case study publication; avoiding the simplifying assumption of a fully mixed pit lake when making predictions; integrating climate change into pit lake predictions; improving the quality of technical reporting; generating industry guidance for pit lake rehabilitation; maximizing opportunities for subaqueous, in-pit disposal of mine wastes; creating a positive legacy through beneficial uses of pit lakes; and verifying predictions using long-term monitoring. Recommended regulatory advancements include: raising expectations of corporate pit lake closure planning and execution; acknowledging good pit lake closure examples; balancing the need to simulate long closure periods with expectations of model reliability; considering the value of pit lakes as future water resources during permitting; and requiring closure costing and bonding commensurate to closure risk.</jats:p

The future direction of pit lakes: part 1, Research needs

AbstractPit lakes are common features of open pit mining and can present significant risks, and yet can also provide beneficial end use opportunities. Many processes that influence the magnitude of these risks and opportunities remains poorly understood, which presents a challenge to pit lake closure and management. In this two-part manuscript, four pit lake subject matter experts from Germany, Canada, Australia, and the USA recommend focus areas for researchers (Part 1) and strategies to structurally improve the practice of pit lake closure for mining industry regulators and corporate sustainability officers (Part 2). In this Part 1, we recommend nine research areas, organized by order of physico-chemical and ecological complexity, where greater understanding of fundamental pit lake processes would lead to improved pit lake management and reuse. Our intent is to guide the direction of emerging and future pit lake research by academic and industry research teams, with funding and oversight from industry and government.</jats:p

A Method to Quantify Wall Rock Mineralogy in an Active Open Pit Mine, and its Application Toward Pit Lake Prediction, Waihi, New Zealand

Subaqueous water-rock reactions that occur at the pit wall/lake water interface can affect the pH of pit lakes. Many pit lake water chemistry models neglect subaqueous reactions, which may create inaccurate predictions over time. A mineral quantification method is presented that defines the wall rock mineralogy of active open pit mines, using samples collected from the Martha Mine, a low-sulfidation epithermal Au-Ag deposit in Waihi, New Zealand. The mineral quantification method has five components: 1. field observations and sampling, 2. X-ray diffraction analyses, 3. selection of representative samples, 4. geochemical analyses, and 5. mineral accounting. Predicted mineral concentrations closely match observed mineral concentrations visually estimated using petrographic techniques. Results of the method can be incorporated into a geochemical model of long-range pit lake water quality resulting from subaqueous water-rock reactions

The Wilkes Land Anomaly revisited

International audienceThe Wilkes Land Gravity Anomaly, first reported in 1959–60, is located in northern Victoria Land in the Pacific Ocean sector of East Antarctica, 1400 km west of the Ross Sea and centred at 70°00'S-140°00'E. Initially described on the basis of ground-based seismic and gravity survey, and estimated at the time to have a diameter of 243 km, the original data are now supplemented by data from airborne radiosound survey, airborne gravity survey, airborne magnetic survey and satellite remote sensing. These new data enable us to expand upon the original data, and reveal that the structure has a diameter of some 510 km, is accompanied by ice streams and a chaotically disturbed region of the continental ice sheet, has a subglacial topographical relief of ≥1500 m, and exhibits a negative free air gravity anomaly associated with a larger central positive free air gravity anomaly. The feature has been described as a volcanic structure, an igneous intrusion, an ancient igneous diapir, a subglacial sedimentary basin, a glacially eroded subglacial valley, a tectonic feature and a meteorite impact crater. We re-examine the feature on the basis of these collective data, with emphasis on the free air gravity anomaly signs, magnitudes and patterns, magnetic signature magnitudes and patterns, and the size, shape, dimensions and morphology of the structure. This enhanced view adds substantially to the original description provided at the time of discovery, and suggests several explanations for the origin of the Wilkes Land Anomaly. However, the importance of this feature lies not only in determining its origin but by the fact that this part of the Wilkes Subglacial Basin is one of the most prominent regional negative geoid and associated gravity anomalies of the Antarctic continent